The need for high temperature materials spurred the development of fiber reinforcing ceramic composites, a now known technique of improving the mechanical properties of ceramic materials. Some of the processes used to create such composites include: (1) pressing powder into a fiber preform, and (2) heating to a form a low viscosity glass, pressing, then crystallizing the glass. However, these fabrication techniques present problems such as fiber fracture due to the high pressures, in the former case, and glass flow at high temperature, in the latter case due to the glass remaining because of incomplete crystallization, both of which weaken the composite.
Chemical vapor deposition (CVD--also known as CVI, chemical vapor infiltration when employed to fabricate composites) has been used to form a ceramic matrix within a fiber preform, providing a possible solution to these problems. However, in the CVD process a problem of voids within the composite arises. (U.S. Pat. No. 3,226,194, incorporated herein by reference, describes a chemical vapor deposition process for applying SiC coatings.) Deposition was found to occur at the surface of the fiber preform sealing the surface and making it eventually impermeable to the reactant gases, and therefore inhibiting the formation of a void free matrix within the preform. The use of a temperature gradient, which induces sequential deposition, from the hottest fiber preform surface to the coolest, helps eliminate voids within the matrix.
Another obstacle to be overcome was that of brittle fracture due to the strong bonding between the fibers and the matrix. In order to obtain a desirably weaker fiber-matrix interface, a thin carbon interlayer was implemented. The carbon interlayer allows energy to be absorbed, resulting in crack deflection or blunting as opposed to brittle fracture. One method of obtaining a carbon interlayer consists of precoating the fibers with a thin layer of pyrolytic carbon by infiltrating the fiber preform with propylene under isothermal conditions, under which propylene will decompose, typically at about 1100.degree. C. Once the formation of the carbon layer is complete, the preform is infiltrated with SiC to form the matrix. ("Fiber-Reinforced SiC Composites with Improved Mechanical Properties", Am. Cerm. Soc. Bull., 66[2] 368(1987)). However, the carbon layer thus formed is difficult to control (thickness) and often too thick.
An objective of this invention is to improve the fracture toughness of a ceramic composite using a single operation to introduce a SiC matrix and a carbon interlayer to a fiber preform.